The compaction equations predict that an instability will
occur if the matrix viscosity decreases with porosity and the system is
subjected to shear. The instability is manifest as a series of bands of high
and low porosity. Porosity bands have been seen in experimental investigations
of sheared partial melt systems and the bands are always oriented roughly 25°
from the direction of maximum compression and occur on length scales similar to
the compaction length and significantly larger than the grain size. Linear instability
analysis of the compaction equations predicts that bands should grow fastest at
the smallest possible length scale and, for purely porosity-dependent matrix
viscosity, parallel to the direction of maximum compression. Various additions
to the matrix rheology law have successfully been used to produce bands with
orientations similar to those seen in experiments, including strain-rate dependent
viscosity, anisotropic viscosity and grain-size and roughness dependent or
damage rheology.
Furthermore, melt bands have been proposed as high permeability
conduits that could channel melt towards the mid-ocean ridge. In order to be effective channels, the bands
must be oriented towards the ridge and their amplitude must be sufficient to
result in a significant permeability variation after evolving through a
mid-ocean ridge corner flow.
In this presentation, I will first present linearized theory
and numerical modeling results for melt band formation in 2D in simple and pure
shear geometries with the rheology laws listed above in order elucidate the
process of melt band formation. I will then present an explanation for the
growth of the width of melt bands to sizes greater than that of the initial
heterogeneity. I will then present linear theory and numerical modeling results
for bands formed when the background velocity field is that of a mid-ocean
ridge corner flow in order to assess the efficacy of melt bands as a channeling
mechanism for melt to mid-ocean ridges. I will show that the rotation of bands
by the mid-ocean ridge flow field causes bands to be poorly oriented to channel
melt to the mid-ocean ridge and that the amplitude of the bands is only likely
to be sufficient to cause the significant permeability heterogeneity that is
necessary to cause channelization if the matrix bulk viscosity is small.

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